Catalysts for the polymerization of olefins comprising either (a) a metallocene-alumoxane complex or (b) a complex formed from transition metal (IVB, VB) and organoaluminum compounds are known in the art. Australian Patent No. 220436 discloses a catalyst comprising the reaction product of a metallocene [bis(cyclopentadienyl)] vanadium salt with a variety of aluminum alkyl compounds. As illustrated by the art, when cocatalyzed with an aluminum alkyl, although a metallocene will exhibit some catalytic activity, the degree of activity is too low to be commercially useful.
The production of a metallocene-based catalyst of commercially useful activity requires the use of an alumoxane as the cocatalyst. Heretofore, the requirement for an alumoxane cocatalyst has entailed extra cost and/or production procedures for preparing a metallocene-based catalyst. An alumoxane is formed from the highly rapid and exothermic reaction of an aluminum alkyl with water. Because of the extreme violence of the reaction the alumoxane component has previously been separately prepared by one of two general methods. In one method, referred to as the "wet solvent production method", extremely finely divided water, such as in the form of a humid solvent, is added to a solution of aluminum alkyl in benzene or other aromatic hydrocarbon. The production of an alumoxane by this process requires use of explosion-proof equipment and very close control of the reaction conditions in order to reduce potential fire and explosion hazards. For this reason, it has been preferred to produce alumoxane by the second method, often referred to as the "hydrated salt method". In this process, an aluminum alkyl is reacted with a hydrated salt, such as hydrated copper sulfate. A slurry of finely divided copper sulfate pentahydrate and toluene is formed and mantled under an inert gas. Aluminum alkyl is then slowly added to the slurry with stirring and the reaction mixture is maintained at room temperature for 24 to 40 hours during which a slow hydrolysis occurs by which alumoxane is produced. Although the production of alumoxane by the hydrated salt method significantly reduces the explosion and fire hazard inherent in the wet solvent production method, production of the alumoxane must nevertheless be carried out separately. Further, before the alumoxane can be used for the production of an active catalyst complex and hydrated salt reagent must be separated from the alumoxane to prevent it from becoming entrained in the catalyst complex and thus contaminating any polymer produced therewith. The process is slow and produces hazardous wastes that create waste disposal problems.
In certain instances wherein a filled polyolefin resin is to be produced the requisite alumoxane cocatalyst may be produced by reacting a triakylaluminum with the filler material then forming the metallocene-alumoxane catalyst complex on the surface of the filler material. For example, U.S. Pat. No. 4,431,788 discloses a catalyst comprising a metallocene complex and starch. These catalysts are produced by a reacting a trialkylaluminum with starch particles having a moisture content below 7 weight percent. The starch partices are then treated with a metallocene to form metallocene-alumoxane catalyst complex on the surface of the starch partices. An olefin is then polymerized about the starch particles by solution or suspension polymerization procedures to form a free-flowing composition of polyolefin-coated starch particles.
Unlike the case of metallocene-based catalysts, the art teaches that the cocatalyst for a catalyst containing a vanadium component (such as VCl.sub.4 or VOCl.sub.3) should be an aluminum akyl. In particular, U.S. Pat. Nos. 4,579,835; 4,607,019 and 4,607,751, disclose the production of a silica gel supported vanadium-based catalyst, and specify that the silica gel must first be dehydrated to remove any water and to reduce the concentration of surface hydroxyl groups which otherwise could react with the aluminum alkyl.
To obtain a degree of productivity useful for commercial production of polyolefins, such prior art supported vanadium catalyst further require the presence of a "promoter" in the polymerization reactor. Halogenated alkanes, such as chloroform or Freon 11, are typically used as a promoter to elevate the productivity of the prior art vanadium based catalyst composition to a level sufficient for commercial production. It is believed that the promoter oxidized the vanadium cation to the higher (III) oxidation state which renders the catalyst its most active, whereas the aluminum alkyl co-catalyst reduces the vanadium cation to the less active (II) oxidation state during polymerization. Therefore, continuous supply of promoter during polymerization has been required to maintain the requisite concentration of vanadium (III) in order to maintain productivity of the catalyst at a useful level.
Consequently, polymerization processes catalyzed by the prior art vanadium-aluminum alkyl complexes suffer several serious disadvantages. Since the prior art vanadium catalysts are effective only in the presence of a promoter, the process of preparing the polyolefins requires extra procedural steps. In particular, the introduction of promoter and vanadium-aluminum catalyst requires distinct steps. Further, the operator must constantly monitor the level of promoter in the reactor in order to obtain efficiency of the prior are vanadium catalyst complex.
The most effective promoters for the prior art vanadium catalyst complexes are, generally, halogenated alkanes. Suitable promoters, such as chloroform and dichlorodifluoroethane, leave an undesired halogen residue in the polymeric end-products. Consequently, use of halogen promoters with the prior art vanadium catalyst complexes may render the polyolefin product highly corrosive or require post polymerization treatment of the polymer product for removal of halogen residue.
Resins produced from the supported vanadium catalysts are typically characterized by a broader molecular weight distribution (MWD), compared to resins made by other transition metal catalysts. Such resins are suitable for application in blow molding.
It would be advantageous to develop a vanadium based supported catalyst useful for the polymerization of olefins which does not require the assistance of a promoter and which exhibits high activity and efficiency. Further, it would be most desirable to produce a catalyst capable of rendering a non-corrosive resin having a broad molecular weight distribution.
It would further be most desirable to devise an economical procedure whereby such catalysts could be safely and economically produced.